Eosinophils are a specific type of white blood cell. It is not required to monitor eosinophils in patients taking clozapine; however, eosinophils often appear on the lab reports done for routine monitoring of neutrophils. Eosinophils can be low or high when patients are taking clozapine. A high count is referred to as eosinophilia. This is more than about 500 eosinophils per uL, the reference range is determined by the laboratory. Eosinophil counts are not useful in predicting side effects of clozapine. However, high eosinophil counts have been associated with parasitic infections, allergic reactions, cancer, Crohn’s disease, and a variety of other disorders. The possibility of these should be considered. When a medical comorbidity is suspected, a referral can be made to primary care or a hematologist. Eosinophil counts greater than 1500 cells per μL have risks associated with them, and the patient should be evaluated medically. Severe eosinophilia is greater than 5000 cells per μL. Counts above 100,000 per μL are associated with vascular problems.